There are three main techniques for on-line payment from a mobile user device:                payment using a bank card, requiring the terminal to be used, on each transaction, to input bank details (card number, expiry date, and pictogram code) for use by a web site or by a mobile application;        payment using an account previously opened with a merchant site or a payment site such as PAYPAL™, ITUNES™, or AMAZON PAYMENTS™, etc., site, that requires bank details, generally bank card details, to be communicated with the merchant or payment site while opening the account, with payment being performed by means of a log-in system and a password that needs to be input on each transaction; and        payment on the bill of a mobile operator, implemented by direct debit on the client's mobile bill from the mobile operator.        
Of those three main techniques, the payment using the mobile bill is particularly appreciated by users because it is simple to use.
That technique of payment by mobile bill operates using a mobile terminal, e.g. a terminal of the “smartphone” type, that is connected to a mobile network. The mobile terminal needs to have a web browser and must previously be registered with a mobile bill payment service of the mobile operator.
Payment for a shopping basket on a merchant web site using a user mobile terminal and using the mobile bill payment technique takes place as follows:
The user uses the mobile terminal to navigate to the merchant site, to prepare a shopping basket containing one or more goods to be purchased, and then clicks on an element of the web page to select the mobile bill method of payment, generally from a plurality of methods of payment made available on the merchant site. After selecting mobile bill payment, the browser of the mobile terminal is redirected by the merchant site to a page of a payment web site.
On being redirected, the mobile terminal sends a request to the payment site, which request contains a payment authorization request containing transaction data (amount, date/time, and transaction identifier). On being transmitted over the mobile network, the payment authorization request is enriched by a platform in the mobile network with the identification data of the mobile terminal, typically the mobile station integrated services digital network (MSISDN) number of the terminal. The MSISDN number or mobile station ISDN number is the publicly known number identifying the mobile telephone in the mobile network. It is this identifier, commonly called the “telephone number”, that needs to be dialed in order to reach the subscriber.
In response to the request, the payment site sends a payment confirmation web page to the terminal, with the terminal displaying the page that contains the transaction data and that invites the user to confirm payment by means of a click (e.g. by clicking on a “confirm” button of the web page). The payment confirmation click triggers execution of the payment transaction. The payment site then transmits the payment authorization request containing the transaction data and the MSISDN number of the mobile terminal to a platform/entity of the mobile operator. On receiving the payment authorization request, the platform of the mobile operator identifies the mobile terminal on the basis of the received MSISDN number, and then verifies that it is registered with the mobile bill payment service and that the amount of the transaction can be authorized. When verification is positive, the platform of the mobile operator responds to the payment site by sending a payment authorization message. The payment site informs the merchant site that the payment transaction has proceeded and uses the mobile network to send a payment confirmation SMS message to the MSISDN number of the mobile terminal, which message indicates that the requested payment has been authorized.
Users appreciate the steps they need to perform when making a payment by means of a mobile bill because they are simple to use. The payment requires only two simple clicks on the part of the user: a first click on the merchant web site and a second click on the payment web site.
The security of such a payment technique relies on the mobile terminal and on the assumption that the user of the mobile terminal has previously activated a security SIM card of the terminal by means of a PIN code for unblocking and possibly unlocking the terminal with the help of an additional unlocking code. Nevertheless, such security is relatively limited. When the SIM card is unblocked and the mobile terminal unlocked, any person other than the owner of the terminal can use the terminal to pay for purchases on the merchant site by using the mobile bill method of payment. As a result, that mobile bill method of payment is generally used only for micropayments, i.e. for payments of small amounts that do not exceed a maximum, which at present is of the order of a few tens of euros.
There therefore exists a need for a technique for on-line payment from a terminal connected to a mobile telephony network that is both simple to use and that presents a higher level of security so as to make it possible to make payments for larger amounts.